Method and device for jetting droplets

ABSTRACT

An ejector for jetting droplets of viscous media onto a substrate is disclosed. The ejector comprises a jetting nozzle having a nozzle space and a nozzle outlet, and an impacting device for impacting a volume of the viscous medium in the nozzle space such that droplets of viscous medium is jetted from the nozzle space through the nozzle outlet towards the substrate. The ejector also comprise a sensor arrangement arranged after the jetting nozzle in the jetting direction, wherein the sensor arrangement is adapted to detect a jetted droplet of viscous medium passing thereby.

TECHNICAL FIELD

The invention disclosed herein relates to jetting of viscous medium ontoa substrate. More precisely, it relates to an ejector for jettingdroplets of viscous medium onto a substrate, as well as a method forjetting such droplets.

BACKGROUND

Ejectors and methods are known in the art for jetting droplets ofviscous medium of fluid, e.g. solder paste or adhesive, onto asubstrate, e.g. a printed wiring board (PWB), thus forming deposits onthe substrate prior mounting components thereon. Such an ejectorgenerally comprises a nozzle space for containing a volume of theviscous medium prior to the jetting thereof, a jetting nozzlecommunicating with the nozzle space, an impacting device for impactingand jetting the viscous medium from the nozzle space through the jettingnozzle in the form of droplets, and a feeder for feeding the medium intothe nozzle space.

The amount, or volume, of the deposited viscous medium at differentlocations of the substrate may be varied by applying several drops ontop of each other, thus forming a larger deposit, or by varying thevolume of the jetted droplet by e.g. feeding a larger or smaller volumeof the viscous medium into the nozzle space.

High production speed and reliability are factors of interest for themanufacturing of e.g. printed circuit board (PCB) assemblies. In orderto increase the production speed, which e.g. can be measured in terms ofthroughput time or mounted components per hour (cph), the application ofviscous medium can be performed “on the fly”, i.e. without stopping foreach location on the substrate where viscous medium is to be deposited.The reliability, such as e.g. the accuracy and repeatability of thejetting process, is of interest due to its effects on the performanceand the quality of the final product, such as e.g. the PCB assembly. Toosmall volumes of deposited medium may e.g. lead to dry joints orloosening components, whereas too large volumes of deposited medium maylead to short-circuiting caused by e.g. solder balls, or defectivecontacts due to contamination of adhesive.

To increase process reliability, an optical or visual inspection such ase.g. manual inspection or Automatic Optical Inspection (AOI) isperformed after the viscous medium has been jetted onto the substrate.The relatively limited ability to detect small variations of volume andthe time consumption and complexity of such inspection is however adrawback associated with such techniques.

Although such an inspection may provide increased process reliability,there is still a need for providing an ejector and method that wouldaddress at least some of the above mentioned issues.

SUMMARY

An object of the technology disclosed is to provide an improved and morereliable and effective ejector for, and method of, jetting droplets ofviscous medium onto a substrate.

This and other objects of the technology disclosed are achieved by meansof an ejector and a method having the features defined in theindependent claims. Different implementations of the technologydisclosed are defined in the dependent claims.

Hence, according to a first aspect of the technology disclosed, anejector for jetting droplets of viscous media onto a substrate isprovided. The ejector comprises a nozzle having a nozzle space and anozzle outlet, and an impacting device for impacting a volume of theviscous medium in the nozzle space such that the volume of the viscousmedium is forced through the nozzle outlet. Thereby droplets of viscousmedium are jetted from the jetting nozzle towards the substrate. Theejector also comprises a sensor arrangement, arranged along a path ofthe jetted droplets of the viscous medium, adapted to detect passage ofthe jetted droplets of the viscous medium towards the substrate.

According to a second aspect of the technology disclosed, a method ofjetting droplets of viscous medium onto a substrate is provided, whereina jetting nozzle comprising a nozzle space and a nozzle outlet isprovided. A sensor arrangement is provided after the jetting nozzle inthe jetting direction, or along a path of the jetted droplets of viscousmedium. The viscous medium is fed into the nozzle space and impactedsuch that the viscous medium is jetted from the nozzle space in the formof droplets through the nozzle outlet towards the substrate. The methodfurther comprises monitoring a sensor parameter reflecting presence ofviscous medium at the sensor arrangement.

The technology disclosed is based on a realisation that by arranging adroplet sensor arrangement between the jetting nozzle and the substrate,onto which the jetted droplets of viscous media is deposited, thejetting characteristics and the jetted droplets can be monitored duringthe jetting process. Information may be obtained, comprising e.g.information on whether droplets are jetted or not due to an impact ofthe impacting device. Thereby missed drops may be detected withoutinspection of the surface of the substrate. If a jetted droplet due toan impact of the impacting device is not verified, the information maybe used for correction of the deposited volume by e.g. addingsupplementary medium to the substrate. The correction may be performedsimultaneously or on the fly, during the present printing process, or inan additional, supplementary printing process. Thereby the need for timeconsuming downstream, posterior inspection of the deposits may bereduced.

The technology disclosed is also advantageous in that it provides arelatively thorough real-time, or instantaneous, monitoring of thevolume of the deposited viscous medium. More specifically, the volumeapplied to a certain location on the substrate may be estimated bycounting the number of the jetted drops that together form the depositedvolume, or by estimating the volume of individual droplets. Thus, thetechnology disclosed enables the volume of the deposited medium to beestimated with an accuracy of the volume of a single jetted drop withoutusing any additional, downstream optical inspection equipment.

In the context of the present application, it is to be noted that theterm “viscous medium” should be understood as solder paste, solder flux,adhesive, conductive adhesive, or any other kind of medium of fluid usedfor fastening components on a substrate, conductive ink, resistivepaste, or the like, and that the term “jetted droplet”, or “shot” shouldbe understood as the volume of the viscous medium that is forced throughthe jetting nozzle and moving towards the substrate in response to animpact of the impacting device. The jetted droplet may also include acluster of droplets jetted due to an impact of the impacting device. Itis also to be noted that the term “deposit”, or a volume of “depositedmedium”, refers to a connected amount of viscous medium applied at aposition on a substrate as a result of one or more jetted droplets, andthat the term “substrate” should be interpreted as a printed wiringboard (PWD), a printed circuit board (PCB), a substrate for ball gridarrays (BGAs), chip scale packages (CSP), quad flat packages (QFP),wafers, flip-chips, or the like.

It is also to be noted that the term “jetting” should be interpreted asa non-contact dispensing process that utilises a fluid jet to form andshoot droplets of a viscous medium from a jetting nozzle onto asubstrate, as to compare to a contact dispensing process, such as “fluidwetting”.

Typically, the ejector is software controlled. The software needsinstructions for how to apply the viscous medium to a specific substrateor according to a predetermined jetting schedule or jetting process.These instructions are called a “jetting program”. Thus, the jettingprogram supports the process of jetting droplets of viscous medium ontothe substrate, which process also may be referred to as “jettingprocess” or “printing process”. The jetting program may be generated bya pre-processing step performed off-line, prior to the jetting process.

Thus, the generation of the jetting program involves importing, to ageneration program, substrate data relating to a unique or predeterminedsubstrate, or a unique or predetermined set of identical substrates; anddefining, on basis of the substrate data, where on the substrate thedroplets are to be jetted. In other words, viscous medium is arranged tobe jetted onto the substrate according to a predetermined jettingprogram.

As an example, a computer program is used for importing and processingCAD data or the like about a substrate. The CAD data may e.g. comprisedata representing position and extension of contact pads, as well asdata representing position, name, and leads of each individual componentthat is to be mounted on the substrate. The program can be used todetermine where on the substrate the droplets are to be jetted, suchthat each component is provided with deposits having the requiredvolume, lateral extension, and/or height. This is a process whichrequires knowledge of the size and volume of a single droplet, how manydroplets that will be sufficient for covering the needs of a specificcomponent, and where on the substrate each droplet should be placed.

When all droplet configurations for all components have been programmed,a jetting path template may be generated, which describes how thejetting nozzle is going to be moved, e.g. by a jetting machine operatingone or more ejectors, in order to jet the droplets of viscous mediumonto the substrate. It is understood that the ejectors may operateconcurrently or consecutively. The jetting path template is transferredto the jetting program which is used for running the jetting machine,and hence the ejector(s), accordingly. The jetting program may alsocomprise jetting parameters, e.g. for controlling the feeding of theviscous medium into the nozzle space, and for controlling the impact ofthe impacting device, in order to provide the substrate with therequired deposits.

The pre-processing step that generates the jetting program may involvesome manual steps performed by an operator. This may e.g. involveimporting the CAD data and determining where on a pad the dropletsshould be positioned for a specific component. It will however berealized that the preprocessing may be performed automatically by e.g. acomputer.

The sensor arrangement may comprise sensor devices configured to usedisturbances of a sensor controlled fields, such as e.g. electromagneticfield controlled by a photosensor, electrical field controlled by acapacitive sensor, and magnetic field controlled by e.g. amagnetoresistive or a hall sensor. The sensor arrangement may also berealised by placing a microphone or pressure sensor in an air knife oran existing gaseous flow. It will be appreciated that the ejector maycomprise further sensor arrangements or sensor devices, such as e.g.temperature or pressure sensors, which may improve the monitoring andcontrol of the jetting process.

A single missed, or absent, shot or a plurality of subsequently missedshots, may be caused by e.g. air voids enclosed in the viscous medium,by a discontinuity in the supply of medium to the nozzle chamber, or adefective ejector. In prior art technology, a substrate having adefective printing result (i.e. having missing deposits or deposits ofwrong or inadequate volume) may be detected after the deposition ofviscous medium, e.g. in a downstream inspection step or during a finaltesting of the product. There is hence a risk that several substrateshaving a defective printing result are processed before the errors aredetected and therefore have to be reworked or discarded.

The present invention is thus advantageous in that it provides thepossibility to monitor the jetting of droplets during the jettingprocess or jetting program such that interruptions or disturbances ofthe jetting process can be detected during the jetting process in realtime or at least early. Thereby potential defects of the printing resultmay be detected prior to forwarding the substrates downstream theprocessing line, which may improve the production yield, reduce therejection rate, and reduce the reworking of substrates.

The technology disclosed is also advantageous in that it provides thepossibility to save additional, downstream inspection steps such as e.g.manual inspection or Automatical Optical Inspection (AOI). Reducing thenumber of tools of the production line, and/or the number operators mayadvantageously reduce production costs.

In-process detection of the jetted drops enables detection of thesmallest applicated amount, i.e. the single jetted drops that form adeposit, prior to application of the viscous medium to the surface ofthe substrate. This enables an improved process control and an improvedmonitoring of the volume of the deposited viscous medium.

The technology disclosed is also advantageous in that it provides thepossibility to correct printing errors by supplemental jetting ofdroplets of the viscous medium onto the substrate without performing aseparate inspection.

A gaseous flow may be provided past an outlet of the jetting nozzle, themagnitude and the velocity of the gaseous flow being sufficient fortransporting viscous medium away from the area at the nozzle outlet withthe gaseous flow. Furthermore, the ejector may be provided with a wall,or vacuum washer, being spaced apart from the nozzle outlet and locatedafter, or downstream, the nozzle outlet seen in the jetting direction.Between the vacuum washer and the nozzle outlet there is formed a spaceacting as a channel or guide for the gaseous flow at and past the nozzleoutlet. The vacuum washer is provided with an opening or orifice,concentric with the nozzle outlet, which allows the jetted droplets topass through the vacuum washer via the orifice. Preferably, the orificeof the vacuum washer also provides an inlet for the gaseous flow towardsand past the nozzle outlet.

According to an implementation of the technology disclosed, the detectoris integrated with the vacuum washer. By e.g. arranging the detector inthe (windswept) wall of the suction hole, or orifice, the gaseous flowmay keep the ejector sufficiently clean from contamination by theviscous medium. This advantageously may enhance the reliability of thejetting process and reduce the risk for interruption or disturbance inthe jetting process due to clogging of any surface of the detector.

Integrating the detector in an existing part of the ejector, such as thevacuum washer, may be advantageous from an ejector production point ofview.

According to different implementations of the technology disclosed, thedetector comprises an optical detector. The detector may furthercomprise a plurality of optical detectors which may be arranged in aplane perpendicular to the path of the jetted droplets, and/or arrangedconsecutively along the path of the jetted droplets. The opticaldetector(s) may comprise a light source, e.g. a light emitting element(LED) emitting light or other electromagnetic energy, and a photosensordetecting the emitted light.

Arranging a plurality of detectors perpendicular to the path of thejetted droplets, e.g. around the orifice of the vacuum washer, mayadvantageously enhance reliability and redundancy of the detection ofpassing droplets.

Furthermore, arranging a plurality of consecutive detectors along thepath of the jetted droplets provides a possibility of detecting thejetted droplets at several positions along their path towards thesubstrate. This may enhance reliability and redundancy, and enablesretrieval of other parameters such as e.g. length and velocity of theshot.

According to an implementation of the technology disclosed, the ejectormay comprise a further detector directed towards the substrate in orderto detect drops of viscous medium on the surface of the substrate, andto measure shape and/or width of the drops and/or the deposits. Thisadvantageously allows for enhanced reliability and redundancy of thejetting process and the detection of potential errors.

According to an implementation of the technology disclosed, the step ofmonitoring comprises calculating at least one presence value (PV) bycomparing at least one sensor value (SV) of the sensor parameter with atleast one reference sensor value (SVref). The sensor value may indicatedisturbances and blocking of the sensor controlled field which isintersected by the path of the jetted droplets. By comparing the sensorvalue (SV) received from the sensor device with a reference sensor value(SVref), or threshold, presence or absence of viscous medium at thesensor arrangement can hence be identified and represented by thepresence value (PV).

According to an implementation of the technology disclosed, the methodfurther comprises the step of calculating a droplet value (DV). Thecalculation includes a comparison between at least two presence values(PV) measured at different times. By comparing two presence values witheach other enables a droplet of viscous medium passing the sensorarrangement to be identified. The identification may e.g. be performedby determining a difference between at least two presence values. Forexample, passage of a droplet may be identified as a first presencevalue indicating presence of viscous medium at the sensor arrangementand a second presence value indicating no presence of viscous medium atthe sensor arrangement. Hence, a droplet value representing a dropletpassing the sensor arrangement may be determined by identifying andanalyzing a difference between presence values.

According to another implementation of the technology disclosed, themethod further comprises calculating a droplet value (DV) by counting atleast two presence values (PV) being equal to or exceeding referencepresence value (PVref) representing presence of viscous medium at thesensor arrangement. The at least two presence values (PV) are measuredat different times, thereby identifying a droplet of viscous mediumpassing the sensor arrangement. A droplet value representing a dropletpassing the sensor arrangement may hence be obtained by identifying andcounting presence values representing presence of viscous medium at thesensor arrangement. Counting a number of presence values (PV)advantageously enables e.g. satellites to be distinguished from the maindroplet.

According to an implementation of the technology disclosed, the methodcomprises monitoring a lapsed time parameter (LTP) reflecting a lapsedtime from the impacting of the viscous medium in the nozzle space to theidentifying of a droplet of viscous medium passing the sensorarrangement, wherein the lapsed time parameter (LTP) including a timevalue (TV). An impact droplet value (IDV) is calculated by comparing thetime value (TV) with a reference time value (TVref). By this comparingthe time value with the reference time value, it may be determined ifthe detected droplet passed the sensor arrangement within a reasonabletime interval from the impact of the impacting interval. Too long timeinterval may indicate that the droplet was not jetted due to the impact,and hence may be regarded e.g. as a missed shot. Analysing the timevalue may advantageously enable a velocity of the droplet to becalculated. The velocity may e.g. be determined by dividing thetravelled distance of the droplet, i.e. the distance between the nozzleoutlet and the sensor device. The time value may also be used formonitoring changes and variations in relative velocity betweenconsecutively jetted droplets. A gradually increasing droplet velocitymay e.g. be detected by correspondingly decreasing time values duringthe jetting process. Similarly, gradually decreasing velocities may beindicated by gradually increasing time values.

Alternatively, or additionally, the impact droplet value (IDV)representing passage of a droplet due to impact of the impacting devicemay obtained by comparing a droplet interval time value (DIV) of adroplet interval time parameter (DTP), which reflects a lapsed timebetween a first and a second droplet of viscous medium passing thesensor, with a reference droplet interval value (DIVref). The DIVref maye.g. be obtained from the jetting program which, as previouslydescribed, may comprise jetting parameters for controlling e.g. theimpacting by the impacting device. By obtaining the lapsed time betweena first and a second shot, and thus the reference droplet interval value(DIVref), it may be determined if a passing droplet was jetted due tothe impact, as expected. If the difference between the droplet intervaltime value and its corresponding reference value is within a referenceinterval, it may e.g. be concluded that the second droplet was jetted inresponse to said impact.

According to an implementation of the technology disclosed, the sensorarrangement comprises at least two sensor devices consecutively arrangedin the jetting direction. According to this implementation, a dropletvelocity value (DVV) is calculated by a comparison between at least afirst presence value (PV) from at least a first sensor device, and atleast a second presence value (PV) from at least a second sensor device,wherein the at least first and second presence values (PV) are measuredat different times. The droplet velocity value, representing a velocityof the droplet at the passage of the sensor arrangement, mayadvantageously be used for as an indicative measure of the quality ofthe printing result and the jetting process. A relatively high dropletvelocity may e.g. affect the printing result in terms of satellites andspreading on the substrate, whereas a relatively low droplet velocitymay lead to irregularly shaped droplets upon impact on the substrate,and hence irregularly shape deposits. The droplet velocity may also beused as an indicative measure of the quality of the viscous medium, e.g.due to the velocity depending on the viscosity and density of theviscous medium. By monitoring the velocity of the jetted droplets, ainstantaneous monitoring of e.g. the quality of the printing result, theperformance of the ejector, and the quality of the viscous medium may beachieved.

Using a sensor arrangement comprising at least two sensor devicesconsecutively arranged in the jetting direction enables a droplet lengthvalue (DLV) to be calculated, wherein the droplet length valuerepresents a length of the jetted droplet in relation to the jettingdirection. The calculation includes comparing the at least firstpresence value (PV) from the at least a first sensor device with the atleast second presence value (PV) from the at least second sensor device,wherein the a least first and second presence values (PV) are measuredat different times.

According to an implementation of the technology disclosed, the sensorarrangement further comprises at least two sensors arranged in a planeperpendicular to the jetting direction. Such arrangement can be used todetermine a droplet volume value (DVOLV) representing a volume of thejetted droplet. The calculation includes calculating a droplet diametervalue (DDIAV) by comparing at least two presence values (PV) from atleast a first and a second sensor device, wherein the at least twopresence values are measured at the same time, and calculating a dropleta droplet length value (DLV) by comparing at least a first presencevalue (PV) from at least a first sensor device with at least a secondpresence value (PV) from at least a second sensor device, wherein the atfirst and second presence values (PV) are measured at different times.The droplet volume value (DVOLV) is then determined based on the dropletlength value (DLV) and the droplet diameter value (DDIAV).

According implementations of the technology disclosed, supplementaljetting of a droplet of viscous medium onto the substrate is performedif a jetted droplet due to impact has not been verified, if the dropletvelocity value (DVV) is below a droplet velocity reference value(DVVref), and/or if the droplet volume value (DVOLV) is below a dropletvolume reference value (DVOLVref).

The supplemental jetting may, e.g., be performed during the presentprinting process. In such case, a jetting module may dynamically modifythe jetting program, which controls the printing process, by adding oneor several additional shots upon detection of one or several missedshots, or droplets having a volume being below a reference volume. Thus,potential errors may be automatically repaired without additional,subsequent processing and without intervention by an operator.

Alternatively, or additionally, the jetting module may prepare a repairjetting program based on information regarding missed shots and/or thevolume of the jetted shots. The repair jetting program may be similar tothe jetting program, i.e. controlling an associated jetting process inorder to jet a required amount of viscous medium onto required positionson the substrate. In the repair jetting program, the required amountsand positions may however be based on the received informationrepresenting missed shots, or shots having low volume.

The repair jetting program may be generated in a similar way as thepreprocessing of the jetting program, as outlined above. The generationmay be performed automatically by e.g. a computer, or involve somemanual steps performed by an operator. The machine may, e.g., displaythe position and/or number of detected missed shots, or deposits havingunexpected volume, and allow for the operator to select which ones ofthe detected errors that should be repaired by additional jetting. Thecomputer may then, based on the input from the operator, prepare arepair jetting program which may be executed by the ejector after thepresent jetting process is finished, or by another, concurrently orsequentially operating ejector.

Correcting the detected error may increase the robustness of the jettingtechnology and thus enhance the quality of the final products. It alsomay reduce the need for downstream inspection and testing of thedeposits and reduce the reworking of the substrates.

Advantageously, the repairing process may be preceded by an automatic ormanual validation of the ejector in response to detected errors, whereinthe quality of the jetting may be verified by e.g. printing a testpattern or jetting droplets at a location beside the surface of thesubstrate.

It will be appreciated that the supplemental jetting may be performed inresponse to, e.g., detection of missed shots, detection of dropletshaving a volume being outside a predetermined reference volume range,and/or detection of droplets having a velocity being outside apredetermined reference velocity interval.

According to implementations of the technology disclosed, the strengthof the impact of the viscous medium in the nozzle space may be increasedif the droplet velocity value (DVV) is equal to or below a dropletvelocity reference value (DVVref). Further, the strength of the impactmay be reduced if the droplet velocity value (DVV) exceeds the dropletvelocity reference value (DVVref).

The velocity of the jetted droplets may affect the quality of theprinting results, inter alia in terms of volume distribution, orspreading, of the droplets on the substrate. High velocity of the jetteddroplets may e.g. result in a relatively flat and wide deposit, while alower velocity might lead to pointed drops having a relatively smalldiameter. Consequently, the distribution of the volume may affect thequality of the final product, wherein e.g. short-circuiting or dryjoints may occur for viscous media comprising solder paste, andloosening components or non-conducting pads may be inherent in viscousmedia comprising e.g. adhesive.

The present implementation is thus advantageous in that is provides apossibility to adjust the velocity of the jetted droplets during thejetting process, and thereby enables final products, such as e.g. PCBassemblies, having a high quality and performance.

According to an implementation of the technology disclosed, the feedingrate of the viscous medium into the nozzle space is increased if thedroplet volume value (DVOL) is equal to or below a droplet volumereference value (DVOLref). Similarly, the feeding rate may be reduced inresponse to the droplet volume value (DVOL) exceeding the droplet volumereference value (DVOLref).

As previously discussed, a deposit having too high volume of e.g. solderpaste may cause short-circuiting, while too high volume of e.g. adhesivemay lead to non-conducting pads. Furthermore, too low volume of e.g.solder paste or adhesive may lead to dry joints or loosening components,respectively. The present implementation is thus advantageous in that isprovides a possibility to adjust the volume of the viscous mediumdeposited on the substrate, and thereby enables e.g. PCB assemblieshaving improved quality and performance.

According to an implementation of the technology disclosed, the methodcomprises providing a substrate sensor arrangement directed towards thesubstrate. A substrate sensor parameter (SSP) reflecting presence ofviscous medium on the substrate is monitored, wherein the substratesensor parameter (SSP) includes a substrate sensor value (SSV). Themethod further comprises calculating at least one substrate presencevalue (SPV), said calculation including a comparison between at leastone substrate presence value (SPV) of said substrate sensor parameter(SSP) with at least one reference substrate presence value (SPVref).Thereby presence of viscous medium on the substrate is identified.

This implementation advantageously provides a possibility to verify ajetted droplet due to an impact of the impacting device, andconsequently enables additional correction of the printing result upondetection of printing errors. Using a substrate sensor arrangement incombination with the sensor arrangement arranged along the path of thejetted drops may improve the reliability and redundancy of to themonitoring and control of the jetted droplets in terms of determininge.g. presence, velocity, diameter, and volume.

The technology disclosed may be embodied as computer-readableinstructions for controlling a programmable computer in such manner thatit performs the method outlined above. Such instructions may bedistributed in the form of a computer-program product comprising acomputer-readable medium storing the instructions.

It will be appreciated that any of the features in the embodimentsdescribed above for the ejectors according to the first aspect of thepresent invention may be combined with the method according to thesecond aspect of the present invention.

Further objectives of, features of, and advantages with, the presentinvention will become apparent when studying the following detaileddisclosure, the drawings and the appended claims. Those skilled in theart will realize that different features of the present invention can becombined to create embodiments other than those described in thefollowing.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of thepresent invention, will be better understood through the followingillustrative and non-limiting detailed description of embodiments of thepresent invention. Reference will be made to the appended drawings, onwhich:

FIGS. 1a-c schematically show a cross sectional side view of a portionof an ejector according to different implementations of the technologydisclosed;

FIGS. 2 and 3 show two possible sensor arrangements for detecting apassing droplet;

FIG. 4 is a perspective view of a vacuum washer comprising a siliconchip and a sensor arrangement;

FIG. 5 is a schematic view of a jetting machine and computer wherein thetechnology disclosed is applicable;

FIG. 6 is a general outline of a method of generating a jetting program;and

FIGS. 7-10 outline a method of jetting droplets of viscous medium onto asubstrate in accordance with an implementation of the technologydisclosed.

All the figures are schematic, not necessarily to scale, and generallyonly show parts which are necessary in order to elucidate the invention,wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

With reference to FIG. 1 a, there is shown a schematic view of anejector according to an implementation of the technology disclosed.

The ejector 1 comprises an impacting device, which in thisimplementation includes a piezoelectric actuator 7 and a plunger 6,which is connected to the piezoelectric actuator 7. The plunger 6 isaxially movable while slidably extending through a bore in bushing 8.Cup springs 9 are provided to resiliently balance the plunger 6 againstthe assembly housing 10, and for providing a preload for thepiezoelectric actuator 7. An eject control unit (not shown) applies adrive voltage intermittently to the piezoelectric actuator 7, therebycausing an intermittent extension thereof, and hence a reciprocatingmovement of the plunger 6 with respect to the assembly housing 10, inaccordance with solder pattern printing data.

Furthermore, the ejector 1 comprises an essentially plate shaped jettingnozzle 2 operatively directed against the substrate 23, onto whichdroplets 22 of viscous medium are to be jetted. In the jetting nozzle23, there is provided a nozzle space 3 and a nozzle outlet 4 throughwhich the droplets 22 are jetted towards the substrate 23. The nozzleoutlet 4 is located at one end, a lower portion, of the nozzle 2. Thenozzle space 3 is arranged for receiving viscous medium, which is forcedthrough the nozzle space 3 and out of the nozzle outlet 4 upon an impactby the plunger 6 of the impacting device.

The impacting device in form of a plunger 6 comprises a piston portionwhich is slideably and axially movably extending through a piston bore,an impact end surface 11 of said piston portion of the plunger 6 beingarranged close to said nozzle 2.

In other implementations of the technology disclosed using a differenttype of ejector(s), the plunger comprising a piston may be replaced byanother type of impacting device such as e.g. a membrane or diaphragm,which may or may not also comprise an ejector control unit adapted toapply a drive voltage intermittently to a piezoelectric actuator inaccordance with what is mentioned above.

All these impacting devices have in common that they are configured toprovide for a non-contact jetting process to form and shoot droplets ofa viscous medium from a jetting nozzle onto a substrate by quicklygenerating a pressure impulse by the reciprocating movement, orvibrating movement, of the impacting device, e.g. a plunger, membrane ordiaphragm.

The impacting devices of the one or more ejector(s) used in connectionwith the technology disclosed may move from a starting position towardsan end position (which may or may not be close to the nozzle of theejector) during a time period of about 1-50 microseconds in order toshoot individual droplets having a deposit volume of between about 100pL and about 30 nL, e.g. about 10 nL or within the size range 5-15 nL.The speed of the impacting device for impacting the jetting nozzle witha pressure impulse may be between about 5 m/s and about 50 m/s.

Hence, the one or more ejector(s) used in connection with the technologydisclosed may be configured to shoot droplets having a deposit volumewith a certain size or size range, e.g. 5-15 nL, 1-5 nL or 10-20 nL. Asmentioned above, the volume of each individual droplet to be jetted ontothe workpiece may be between about 100 pL and about 30 nL and the dotdiameter for each individual droplet may be between about 0.1 mm andabout 1.0 mm.

The upper surface of the nozzle 2 is positioned opposite to the impactend surface. Axial movement of the plunger 6 towards the nozzle 2, saidmovement being caused by the intermittent extension of the piezoelectricactuator 7, will cause a rapid pressurization and jetting through thenozzle outlet 4 of any viscous medium contained in the nozzle space.

Viscous medium is supplied to the nozzle space 3 a supply container, viaa feeder 12. The feeder 12 comprises an electric motor (not shown)having a motor shaft 13 partly provided in a tubular bore, which extendsthrough the ejector housing 10 to an outlet port communicating with thenozzle space. An essential portion of the rotatable motor shaft, or feedscrew 13, is surrounded by a tube 14, made of an elastomer or the like,arranged coaxially therewith in the tubular bore, the threads of therotatable feed screw 13 making sliding contact with the innermostsurface of the tube. An electronic control signal provided by a supplycontrol unit (not shown) to the motor causes the feed screw 13 to rotatea desired angle, or at a desired rotational speed. Viscous mediumcaptured between the threads of the rotatable feed screw 13 and theinner surface tube are then made to travel from the inlet port to thenozzle space 3 in accordance with the rotational movement of the feedscrew 13, thereby feeding viscous medium into the nozzle space 3.

A sensor arrangement 5 is arranged after the jetting nozzle 2, as seenin the direction of the jetted droplet 22, such that the path of thejetted droplet 22 intersects a sensor field 17 controlled by the sensorarrangement 5. Thus, the droplet 22 passing by the sensor arrangement 5may cause a disturbance of the sensor controlled field 17 such that apresence of viscous medium may be detected.

With reference to FIG. 1 b, there is depicted an ejector 1 similar tothe ejector as described with reference to FIG. 1 a. According to FIG. 1b, the ejector may further comprise a wall, or vacuum washer 24,arranged below, or after, the nozzle outlet 4, as seen in the jettingdirection. The vacuum washer 24 is provided with a through hole, ororifice, through which the jetted droplet 22 may pass without beinghindered or negatively affected by the vacuum washer 24. Consequently,the hole is concentric with the nozzle outlet 4. The vacuum washer 24 isspaced apart from the nozzle outlet 4 such that an air flow chamber 16is formed between the vacuum washer 24 and the nozzle outlet 4, actingas a channel or guide which enables a gaseous flow towards and past thenozzle outlet 4.

FIG. 1c depicts a further ejector 1 similar to the ejectors aspreviously described with reference to FIGS. 1a and b. As indicated inFIG. 1 c, the sensor arrangement 5 may be integrated with the vacuumwasher 24.

With reference to FIG. 2, a jetting nozzle 2, a piston 6, and a sensorarrangement 5 is depicted in accordance with an implementation of thetechnology disclosed. The jetting nozzle 2 comprises a nozzle space 3provided with a volume of viscous medium, which, upon impact by theimpacting device, is forced through the nozzle outlet 4. Thereby ajetted droplet 22 of the viscous medium is expelled from the jettingnozzle 2 and passing through an optical field 17 controlled by thesensor arrangement, comprising e.g. an optical sensor. The droplet 22passing by the sensor arrangement 5 may cause a disturbance of thesensor controlled field 17, such that a presence of viscous medium maybe detected.

A similar arrangement as described with reference to FIG. 2 is shown inFIG. 3, wherein a first and second sensor arrangement 5 a, 5 b isconsecutively arranged in the jetting direction. The path of the jetteddroplet hence intersects two sensor controlled fields 17 a, 17 b, suchthat at least two different and time separated sensor signals may begenerated upon passage of the droplet 22. A substrate sensor arrangement5 c is directed towards the substrate 23 so as to enable detection ofviscous medium on the substrate 23.

It will however be appreciated that the sensor arrangement 5 maycomprise a plurality of sensor devices consecutively arranged in thejetting direction, which may be integrated with a vacuum washer 24 ornot integrated with the same.

Turning now to FIG. 4, the vacuum washer 24 may comprise a silicon chip21 having a suction hole 15 and a sensor device 5 arranged across thesuction hole 15, wherein the sensor 5 device includes a light emittingdiode (LED) 17 and an oppositely arranged photo sensor 18. The LED 17and the photo sensor 18 are connected to electric wirings 19 fortransferring electric power and sensor signals to and from thesurroundings via electric contact pads 20. The vacuum washer 24 and theintegrated sensor arrangement may be combined with any one of theembodiments as described with reference to FIG. 1-3.

With reference to FIG. 5, there is illustrated a jetting machine 51 inwhich substrates 57 will be provided with droplets of viscous medium. Asoftware program is run on a computer 53, which communicates with themachine 51. The software program has a database, which holds principalmanufacturing data about substrates, e.g. PCBs, machine data for themachine in which the substrates are to be processed. Substrate data 55about the substrate is imported to the database, preferably in the formof CAD data comprised in a CAD file. The program is adapted forgenerating a jetting program controlling the jetting process. Thesoftware program is available off-line in order for an operator to beable to work with the jetting program generation without interferingwith any simultaneous running of the machine control software which isto be provided with the jetting program. The software program may beprovided on a computer readable medium which is illustrated by a CD ROM59 in FIG. 5.

The jetting program for a specific machine, or a plurality of machinesthat will use the same jetting program, may be generated as follows.First the operator, working on the computer 53 where the softwareprogram has been loaded, on basis of the CAD data for a substrate,assigns the components that are to be places on the substrate to themachine by means of the software program. Component data about thecomponents, such as their extension, regarding the housing as well asthe leads, if any, and their position on the substrate, is comprised inthe substrate data. By opening the machine interface for the presentmachine on the computer, the operator may begin the procedure ofgenerating data for the jetting program based on the substrate data.

FIG. 6 is a flowchart illustrating an example of a pre-processing stepthat generates a jetting program for controlling the jetting process. Ina first step 601, the CAD data for a substrate is imported into thesoftware program for off-line pre-processing, wherein the CAD data, inthe next step 602, is converted into assembly data which e.g. maydescribe position and extension of each individual component that is tobe assembled. In the next step, the required deposits are defined 603and assigned to their respective pad or position on the substrate. Oncethe required deposits are defined 603, the information is compiled 604into a jetting program which is sent 605 to the jetting machine whereinit may be executed so as to control the jetting process. The jettingprogram may comprise data for controlling e.g. travelling paths of theejector(s), and jetting parameters controlling the impact of theimpacting device and the feeding of the viscous medium into the nozzlespace. Thereby the jetting of droplets of viscous medium may becontrolled such that the required deposits are provided.

A repair jetting program, in which printing errors such as e.g. missedshots and droplets having a volume below a predetermined value, may begenerated similarly to the jetting program as described with referenceto FIG. 6. Upon detection of printing errors, e.g. by use of the sensorarrangement referred to above, a repair jetting program may be generatedby defining 603 required deposits based on the detected errors. Therepair jetting program may then be compiled 604 and sent 605 to thejetting machine wherein the missing droplets, or erroneous deposits, arecomplemented by additional jetting. It will be realised that thepre-processing of the repair jetting program may be performedautomatically, e.g. by the software program, of include some manualsteps performed by an operator.

With reference to FIG. 7, there is shown a general outline of a methodof jetting droplets onto a substrate 23 according to an implementationof the technology disclosed.

According to this embodiment, a jetting nozzle 2 comprising a nozzlespace 3 and a nozzle outlet 4 is provided 102. After the jetting nozzle2, in the jetting direction, a sensor arrangement 5 is provided 103,which e.g.

comprises an optical sensor device 17, 18. Viscous medium, such as e.g.solder paste, is fed 106 into the nozzle space 3 and impacted 108 by animpacting device such that the viscous medium is jetted from the nozzlespace 3 in the form of droplets 22 through the nozzle outlet 4 towardsthe substrate 23. The method further comprises a step of monitoring 110a sensor parameter reflecting presence of viscous medium at the sensorarrangement 5.

As shown in FIG. 8, the monitored sensor parameter may comprise a sensorvalue (SV) which indicates presence of viscous medium at the sensordevice. Hence, the method as described with reference to FIG. 7 comprisea step of calculating 112 a presence value (PV) identifying the presenceof viscous medium at the sensor arrangement. The calculation of thepresence value may include a comparison between a sensor value of thesensor parameter and a reference sensor value (SVref). The SVref maye.g. be a threshold indicating whether the sensor value representspresence of viscous medium or not. The presence value (PV) may e.g. be abinary indicator, wherein “1” may define presence of viscous medium and“0” defines absence of viscous medium.

In the next step, a droplet value (DV), identifying a droplet of viscousmedium passing the sensor arrangement, is calculated 114 by e.g.comparing at least two presence values (PV) measured at different times.This may for example be achieved by comparing two presence valuesconsecutively registered by the same sensor device. A first PVrepresenting presence of viscous medium, followed by a second PVrepresenting absence of viscous medium, may e.g. indicate that a dropletwas passing the sensor device. The calculation may also comprise acomparison of several presence values in order to improve thereliability of the identification and to reduce noise of themeasurements.

It will also be realised that passage of a droplet may be identified inseveral other ways readily understood by a person skilled in the art.For example, the droplet value may be calculated 114 by counting atleast two presence values (PV) being equal to or exceeding a referencepresence value representing presence of viscous medium at the sensorarrangement.

Further, the passing droplet may be verified as a jetted droplet, i.e.an intentional droplet passing the sensor arrangement due to an impactof the impacting device. This may be achieved by monitoring 116 a lapsedtime parameter (LTP) and calculating 118 an impact droplet value (IDV).The lapsed time parameter reflects a lapsed time from the impacting 108of the impacting device to the identifying 114 of a droplet passing bythe sensor arrangement, and the impact droplet value may be calculated118 by comparing a time value (TV) of the lapsed time parameter (LTP)with a reference time value (TVref). A relatively low time value maye.g. indicate that the passing droplet is passing the sensor arrangementdue to the recent impact, whereas a relatively high time value mayindicate that the droplet is not passing the sensor arrangement due tothe impact.

As shown in FIG. 9, the method may further comprise a step ofcalculating 120 a droplet velocity value (DVV) by means of a timeinterval defined by the passage of the droplet between at least twosensor devices consecutively arranged in the jetting direction. Thecalculation 120 includes a comparison between a first presence value(PV) from a first sensor device, and a second presence value (PV) form asecond sensor device, which thereby provides the time intervalrepresenting the time it takes for the droplet to travel the distancebetween the first and the second sensor device. By dividing the distancebetween the sensors in the jetting direction with the timer interval, anaverage droplet velocity may be obtained.

By using at least two sensor devices consecutively arranged in thejetting direction, both a droplet velocity value (DVV) and a dropletlength value (DLV) may be calculated 120, 122. The droplet velocityvalue (DVV), obtained by comparing a first presence value (PV) from afirst sensor device with a second presence value (PV) from a secondsensor device, may be used together with a third presence value fromeither one of the sensor devices to determine the droplet length value(DLV). The first presence value may e.g. represent a front of thedroplet, and the third presence value, e.g. obtained from the firstsensor device, an end of the droplet. Based on the time interval betweenthe passing of the front and the end of the droplet with the dropletvelocity value (DVV), the length—i.e. the distance between the front andthe end—of the droplet may be calculated 122.

A sensor arrangement further comprising at least two sensor devicesarranged in a plane perpendicular to the jetting direction. By comparingtwo presence values (PV) from a first and a second sensor devicearranged in a plane perpendicular to the jetting direction, wherein thetwo presence values, the diameter of the droplet may be calculated as adroplet diameter value (DDIAV). A droplet volume value (DVOLV) may thenbe calculated 126 based on the droplet diameter value (DDIAV) and thedroplet length value (DLV).

Supplemental jetting 128 of a droplet of viscous medium onto thesubstrate may be performed if a jetted droplet due to impact has notbeen verified 118, if the jetted droplet has a too low velocity, or ifthe jetted droplet has a too low volume. The additional jetting 128 maye.g. be performed in a separate, correcting printing process, orperformed dynamically during the jetting the jetting process.

If the calculated 120 droplet velocity is below a reference dropletvelocity value, a step of increasing 130 a strength of the impact of theviscous medium may be performed so as to increase the droplet velocity.Correspondingly, a step of increasing 132 the strength to the impact mayperformed in response to the calculated 120 droplet velocity being equalto or exceeding the reference droplet velocity value. The adjustment130, 132 of the impact strength may e.g. be achieved by modifying theapplied voltage to the piezoelectric actuator connected to the piston.

In response to a droplet volume value being too low or high, comparedwith a reference droplet volume value, the method may comprise a step ofincreasing 134 or decreasing 136, respectively, the feeding rate of theviscous medium into the nozzle space. This may e.g be performed byadjusting the speed of the electric motor operating the feeding screw.

Finally, FIG. 10 shows a method similar to the method as described withreference to FIG. 7, further comprising the step of providing 105 asubstrate sensor arrangement directed towards the substrate, monitoring111 a substrate sensor parameter (SSP) reflecting presence of viscousmedium on the substrate, and calculating 113 a substrate presence value(SPV). The calculation includes comparing a substrate presence value(SPV) of the substrate sensor parameter (SPV) with a reference substratepresence value (SPVref) and thereby indentifying presence of viscousmedium on the substrate.

It will be appreciated that any one of the embodiments described abovewith reference to FIGS. 1-4 is combinable and applicable to the any oneof the embodiments of the method described herein with reference toFIGS. 7-10.

As outlined above, the method illustrated by FIGS. 7-10 may be embodiedas computer-executable instructions distributed and used in the form ofa computer-program product including a computer-readable medium storingsuch instructions. By way of example, computer-readable media maycomprise computer storage media and communication media. As is wellknown to a person skilled in the art, computer storage media includesboth volatile and non-volatile, removable and non-removable mediaimplemented in any method or technology for storage of information suchas computer readable instructions, data structures, program modules orother data. Computer storage media includes, but is not limited to, RAM,ROM, EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (DVD) or other optical disk storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices.Further, it is known to the skilled person that communication mediatypically embodies computer readable instructions, data structures,program modules or other data in a modulated data signal such as acarrier wave or other transport mechanism and includes any informationdelivery media.

While specific embodiments have been described, the skilled person willunderstand that various modifications and alterations are conceivablewithin the scope as defined in the appended claims.

1.-30. (canceled).
 31. A method of jetting droplets of viscous mediumonto a substrate, the method comprising the steps of: providing ajetting nozzle comprising a nozzle space and a nozzle outlet; providinga sensor arrangement after the jetting nozzle in the jetting direction;feeding said viscous medium into the nozzle space; impacting saidviscous medium in the nozzle space, thereby jetting viscous medium fromthe nozzle space in the form of droplets through the nozzle outlettowards the substrate; monitoring a sensor parameter reflecting presenceof viscous medium at the sensor arrangement; and generating a repairjetting program based on information detected by said sensorarrangement, wherein said repair jetting program is adapted to controlan associated jetting process in order to jet a required amount ofviscous medium onto required positions on said substrate, and whereinsaid associated, or supplemental, jetting process is performed inresponse to at least one of detection of missed shots, detection ofdroplets having a volume being outside a predetermined reference volumerange, and/or detection of droplets having a velocity being outside apredetermined reference velocity interval.
 32. The method according toclaim 31, wherein said step of monitoring comprises: calculating atleast one presence value (PV), said calculation including a comparisonbetween at least one sensor value (SV) of said sensor parameter with atleast one reference sensor value (SVref), thereby identifying thepresence of viscous medium at the sensor arrangement.
 33. The methodaccording to claim 32, said method further comprising the step of:calculating a droplet value (DV), said calculation including acomparison between at least two presence values (PV) measured atdifferent times, thereby identifying a droplet of viscous medium passingthe sensor arrangement.
 34. The method according to claim 32, saidmethod further comprising the step of: calculating a droplet value (DV),said calculation including counting at least two presence values (PV)being equal to or exceeding reference presence value (PVref)representing presence of viscous medium at the sensor arrangement, saidat least two presence values (PV) being measured at different times,thereby identifying a droplet of viscous medium passing the sensorarrangement.
 35. The method according to claim 33, further comprising:monitoring a lapsed time parameter (LTP) reflecting a lapsed time fromthe impacting of said viscous medium in the nozzle space to theidentifying of a droplet of viscous medium passing the sensorarrangement, said lapsed time parameter (LTP) including a time value(TV); calculating an impact droplet value (IDV), said calculationincluding comparing the time value (TV) with a reference time value(TVref), thereby verifying jetting of a droplet due to said impacting.36. The method according to claim 33, further comprising: monitoring adroplet interval time parameter (DTP) reflecting a lapsed time between afirst droplet of viscous medium passing the sensor arrangement and asecond droplet of viscous medium passing the sensor arrangement, saiddroplet interval time parameter (DTP) including a droplet interval value(DIV); calculating an impact droplet value (IDV), said calculationincluding comparing the droplet interval value (DIV) with a referencedroplet interval value (DIVref), thereby verifying jetting of dropletdue to impacting of the impacting device.
 37. The method according toclaim 35, wherein the sensor arrangement comprises at least two sensordevices consecutively arranged in the jetting direction, the methodfurther comprising the step of: calculating a droplet velocity value(DVV), said calculation including a comparison between at least a firstpresence value (PV) from at least a first sensor device, and at least asecond presence value (PV) from at least a second sensor device, whereinthe at least first and second presence values (PV) are measured atdifferent times.
 38. The method according to claim 35, wherein thesensor arrangement comprises at least two sensor devices consecutivelyarranged in the jetting direction, the method further comprising thestep of: calculating a droplet length value (DLV), said calculationincluding a comparison between at least a first presence value (PV) fromat least a first sensor device, and at least a second presence value(PV) from at least a second sensor device, wherein the a least first andsecond presence values (PV) are measured at different times.
 39. Themethod according to claim 38, wherein the sensor arrangement furthercomprises at least two sensor devices arranged in a plane perpendicularto the jetting direction, the method further comprising the step of:calculating a droplet diameter value (DDIAV), said calculation includinga comparison between at least two presence values (PV) from at least afirst and a second sensor device, wherein the at least two presencevalues are measured at the same time; calculating a droplet length value(DLV), said calculation including a comparison between at least a firstpresence value (PV) from at least a first sensor device, and at least asecond presence value (PV) from at least a second sensor device, whereinthe at first and second presence values (PV) are measured at differenttimes; and calculating a droplet volume value (DVOLV) based on saiddroplet length value (DLV) and said droplet diameter value (DDIAV). 40.The method according to claim 35, further comprising a step of:performing supplemental jetting of a droplet of viscous medium onto thesubstrate if a jetted droplet due to impact has not been verified. 41.The method according to claim 37, further comprising the step of:performing supplemental jetting of a droplet of viscous medium onto thesubstrate if said droplet velocity value (DVV) is below a dropletvelocity reference value (DVVref).
 42. The method according to claim 39,further comprising the step of: performing supplemental jetting of adroplet of viscous medium onto the substrate if said droplet volumevalue (DVOLV) is below a droplet volume reference value (DVOLVref). 43.The method according to claim 40, wherein the supplemental jetting isperformed during the first jetting process.
 44. The method according toclaim 40, wherein the supplemental jetting is performed after the firstjetting process.
 45. The method according to claim 40, wherein thesupplemental jetting is performed by an additional ejector.
 46. Themethod according to claim 37, further comprising: increasing a strengthof the impact of said viscous medium in the nozzle space if the dropletvelocity value (DVV) is equal to or below a droplet velocity referencevalue (DVVref).
 47. The method according to claim 37, furthercomprising: reducing the strength of the impact of said viscous mediumin the nozzle space if the droplet velocity value (DVV) exceeds thedroplet velocity reference value (DVVref);
 48. The method according toclaim 39, further comprising: increasing a feeding rate of the viscousmedium into the nozzle space if the droplet volume value (DVOL) is equalto or below a droplet volume reference value (DVOLref).
 49. The methodaccording to claim 39, further comprising: reducing the feeding rate ofthe viscous medium into the nozzle space if the droplet volume value(DVOL) exceeds the droplet volume reference value (DVOLref).
 50. Themethod according to claim 31, further comprising: providing a substratesensor arrangement directed towards the substrate; monitoring asubstrate sensor parameter (SSP) reflecting presence of viscous mediumon the substrate, said substrate sensor parameter (SSP) including asubstrate sensor value (SSV); calculating at least one substratepresence value (SPV), said calculation including a comparison between atleast one substrate presence value (SPV) of said substrate sensorparameter (SSP) with at least one reference substrate presence value(SPVref), thereby identifying the presence of viscous medium on thesubstrate.
 51. The method according to claim 31, wherein said generatedrepair jetting program is automatically generated.
 52. The methodaccording to claim 31, wherein said generated repair jetting program isexecuted by another, concurrently operating ejector.
 53. The methodaccording to claim 31, wherein said generated repair jetting program isexecuted by another, sequentially operating ejector.
 54. A method for ajetting process for jetting droplets of viscous medium onto a substrate,the method comprising the steps of: providing a jetting nozzlecomprising a nozzle space and a nozzle outlet; providing a sensorarrangement after the jetting nozzle in the jetting direction; feedingsaid viscous medium into the nozzle space; impacting said viscous mediumin the nozzle space, thereby jetting viscous medium from the nozzlespace in the form of droplets through the nozzle outlet towards thesubstrate; monitoring a sensor parameter reflecting presence of viscousmedium at the sensor arrangement; and modifying a jetting program whichcontrols a printing process, wherein said modifying of said jettingprogram is performed during said printing process by adding one orseveral additional shots to said jetting program in response to saidmonitoring of said sensor parameter by the detection of at least one ofmissed shots, droplets having a volume being outside a predeterminedreference volume range, and/or droplets having a velocity being outsidea predetermined reference velocity.